Figure 1. A winter wheat field infected with Wheat Streak Mosaic Virus in Central South Dakota 2017.
About Wheat Streak Mosaic Virus
Wheat streak mosaic disease is caused by viral pathogen called Wheat Streak Mosaic Virus (WSMV). This virus is transmitted by microscopic mites called Wheat Curl Mites (WCM). WCMs can only be seen under magnification (Figure 2). WCMs move from field to field by wind. WCMs are not capable of moving on their own since they do not develop wings but can crawl to neighboring plants. WSMV can result in severe yield losses and sometimes an entire wheat field can be lost when wheat is sprayed out and a different crop planted in spring.
Figure 2. Wheat curl mites under magnification.
Common Producer Questions
Many questions were raised on wheat streak mosaic disease of wheat during the recent Ag Horizon Conference. This was, in part, due to the widespread occurrences of this disease during the 2017 season (Figure 1). Some of the questions raised included:
Can infections spread during the winter?
On the question of whether WSMV can continue to spread during winter period, the WCMs are active when the wheat plant is actively growing. Since winter wheat is dormant at this time, the WCMS will be inactive and no further spread of WSMV happens during this time of the year. WCMs resume to be active when temperatures warm up in spring. It should be mentioned here that infections, which take place in the fall, are the most damaging to wheat. Spring infections cause mild grain yield loss.
Can wheat curl mites survive the winter?
Wheat curl mites can survive bitter cold winter temperatures. WCMs overwinter as eggs, immature or as adult mites. The adult WCMs reside near the growing point of a wheat plant and can be insulated from low temperatures especially under snow cover.
Can insecticides be used to control wheat curl mites?
Insecticides are not effective against the WCMs curl mites mainly because WCMs are protected from exposure to insecticides as they inhabit the inner whorl of the leaf near the growing point of the plant. Insecticides used to control other types of mites such as spider mites will not control WCMs.
Could neighboring pasture grasses be a source of inoculum?
Neighboring pasture grasses can be a source of the WCMs and the virus. However, the preferred host for the vector and the virus is wheat. Therefore while pasture grasses may serve as a source of inoculum, this inoculum is limited and usually a few wheat plants along the field edges will be affected. Larger epidemics of this disease happen when the inoculum is from within the field or when WCMs are blown in from a neighboring wheat fallow during fall shortly after planting.
Can it spread through seed?
Seed is not an important source of inoculum for WSMV. Although research has shown that a low percentage (<0.5%) of seed harvested from infected plants can transmit WSMV, the infected seedlings do not lead to significant wide spread disease within the field. WCMs remain the major source of WSMV.
The best time to send a sample for confirming WSMV is spring once wheat has resumed growing. It is important to confirm presence of WSMV before management decisions such as spraying out wheat and planting something else are made. General yellowing of plants should not be solely taken as indicator of WSMV as other factors such as nitrogen deficiency, chloride deficiency, and water logging can cause wheat plants to look yellow. When taking samples of wheat for WSMV testing, obtain at least five samples on a transect across the field in the direction of the prevailing wind. This will help to gauge the extent of WSMV spread across the field.
Once plants are infected with a viral disease, nothing can be done to ‘cure’ the plants of the virus. WSMV management requires pre-planting practices that prevent/limit infection from taking place in the fall. The best practice to manage WSMV is to destroy the volunteer wheat and grass weeds (“green bridge”) at least two weeks before planting. WCMs cannot survive more than 48 hours without a living green tissue to feed on. The second practice is to delay planting in fall especially following a year when WSMV was wide spread in an area. Once wheat plants escape fall WSMV infections, infections that happen in spring do not result in significant yield loss. A few cultivars are resistant or tolerant to WSMV. See the 2017 South Dakota Winter Wheat Variety Trial Results for ratings for WSMV.